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We investigated the valley depolarization and carrier relaxation process ofmonolayer and bilayer WS2 at 10 K by using time-resolved Kerr rotation (TRKR)and differential reflectance measurement simultaneously. Two decay processesextracted from TRKR signals were observed on both monolayer and bilayer WS2. Inmonolayer WS2, the initial ultrafast decay component (less than 1 ps) wasinterpreted as the stimulated emission or Pauli-blocking of electrons and holesin one valley by comparing with carrier decay process. The relatively slowcomponent was around 4 ps under low excitation energy (less than 2.21 eV) andthen increases with excitation energies, which approaches a saturation value of15 ps. The onset excitation energy of 2.21 eV suggests the ${\Gamma}$-valleyassisted intervalley scattering between the K and K' valley play a criticalpart during the valley depolarization process under the off-resonanceexcitation condition. By contrast, the slow decay component (48 ps) of bilayerWS2 is comparable with the carrier lifetime (58 ps). It is attributed toirreversible scattering processes from K (or K') to ${\Gamma}$ valley due toits characteristic of indirect band gap semiconductor.

${\Gamma}$-valley assisted intervalley scattering on monolayer and bilayer WS2 revealed by time-resolved Kerr rotation spectroscopy